The demand for higher reactor throughput continues to increase for a variety of industrial processes linked to oil and gas recovery, alternative fuel production, sustainability of the environment and process emissions. Such demands are partially driven by the ever-increasing cost of fuel and the need for various chemical feed stocks. One example is the demand for larger cryogenic air separation units (ASUs) to meet the growing needs for large quantities of oxygen and nitrogen used in various industrial process industries. ASUs require front end purification reactors (adsorption vessels) to purify the feed air stream by removing carbon dioxide, water, trace hydrocarbons and other contaminants prior to entering the ASU. Larger ASUs require larger “prepurification units”, as they are commonly known; to treat the incoming feed air prior to a cryogenic distillation process. This presents a challenge to reactor designers when trying to control the size of the reactor since higher throughput of feed air demands a proportional increase in the frontal flow area provided by the vessels resulting in larger, more costly vessels.
Gas purification, separation or reaction processes using active materials such as adsorbents and/or catalysts are well known in the art and there are several reactor vessel designs in use today for these types of processes. Examples include both vertically and horizontally oriented cylindrical vessels with upward air flow through the bed of adsorbent material or reactant and/or catalytic material during purification, separation or chemical reaction. A third type of vessel, as employed herein, is oriented with a vertical central or longitudinal axis and an internal design that directs the process gas flow radially through the bed. This radial flow design consists of a pressure vessel enclosing gas permeable concentric inner and outer baskets to contain a bed of one or more layers of active material. Such radial flow designs offer the ability to increase frontal flow area by increasing the height of the vessel without substantially altering the vessel footprint (ground area requirements). Furthermore, radial flow designs offers a more efficient means of increasing flow area compared to either horizontal or axial flow reactor designs.
Radial flow reactors typically operate continuously or in cyclic mode, depending upon the gas treatment process. Many processes, such as adsorption processes, operate cyclically in either pressure swing (PSA), vacuum swing (VSA), temperature swing (TSA) mode or in combinations of these modes wherein one or more components of the feed stream are adsorbed during the adsorption step and then desorbed or otherwise flushed from the adsorbent during the regeneration step. Thermal variations accompanying these cyclical processes, such as in TSA processes, affect bed and vessel components. Internal components, depending upon their configuration as well as their manner of connection to the vessel, expand and contract when exposed to temperature variations and thus experience loads induced by these temperature changes. Such thermally induced loads create significant mechanical stresses on all elements of the internal basket assemblies, the magnitude of such induced loads increases with increasing temperature difference. Axial and radial displacement of the basket walls may also result in compression of the bed of active material and the material particles may migrate or be damaged as a result of the basket wall movement and especially when such materials are loosely packed. In the worst case, these effects can cause physical breakdown of the active material and/or mechanical failure of the basket assemblies.
Free-flowing active particulate material is typically loaded into a bed by methods such as pouring, dumping or “sock loading,” creating a loosely and non-uniformly packed bed with excess voids between the particles. Beds loaded by these techniques are subject to as much as 10% or more volume reduction by the settling of the particles. Such settling is made possible by the excess void volume and is promoted by a combination of the cycling of the flow and temperature, the expansion and contraction of the baskets and normal gravitational forces. It is desirable to mitigate these effects by maximizing the packing density and at the same time minimizing excess void volume. It is thus preferred to load a vessel in a manner that results in a uniform and densely packed bed of active material(s) wherein the potential for settling is minimized or even eliminated. This method is known as “dense loading” or “dense packing” and is also referred herein as “dense load” or “dense pack.” Potential benefits of dense loading include increased reactor capacity or throughput, improved yield and/or product quality and elimination of hot spots. Furthermore, automated dense loading is safer as it eliminates having operators inside the reactor during loading.
It is further desirable to load multiple discrete radial layers of different active materials simultaneously. Such loading methods are generally known for radial flow vessels utilized for PSA processes, for example see U.S. Pat. No. 5,836,362. In such processes there are no significant induced thermal loads. The internal basket structure of such prior art vessels is designed such that the inner basket is not directly attached to the upper head of the vessel. As a result, the loading method described above is facilitated wherein a rotating arm or arms may extend from the central axis of the vessel (and baskets) to the inside wall of the outer basket. The arms are free to continuously sweep the entire 360 degrees of the annular space between the baskets as the active material(s) is loaded the form the bed. Such a loading method cannot be readily applied to vessels designed for thermal cycling wherein the inner basket is affixed or otherwise connected to the top head of the vessel, i.e. the free rotation of arms about the vessel central axis is prevented by the presence of the continuously extended inner basket. Thus, the first problem to address is the desire to dense load active material into a radial flow reactor designed for thermal cycling processes wherein an inner basket is rigidly and continuously attached to the top head of the vessel.
Radial flow reactors typically require multiple layers of active materials. For example, multiple adsorbent layers are used in air prepurification processes, e.g. alumina to primarily remove the H2O and molecular sieves to primarily remove the CO2, to reduce energy consumption by decreasing the maximum regeneration temperature required and/or by decreasing the amount of regeneration gas required. Additional layers of adsorbents, catalysts, or other active material may also be required when other contaminants must be removed, such as contaminants for which the primary active materials in the bed have no selectivity, capacity or reactivity.
In order to accommodate multiple layers of materials, multiple baskets have been employed. When using more than two structural baskets, both the fabrication of the vessel and the loading of active material(s) become significantly more complex and more costly. Furthermore, rigidly affixed internal baskets transfer additional stresses to the bed containing the active materials due to the induced thermal loads on these internal baskets. Thus, a second problem to address is the need to eliminate additional baskets between the inner most and outer most baskets.
Thus, there is significant motivation to improve the mechanical design of radial flow reactors to affect greater operational reliability, lower cost and increased process flexibility while still limiting the overall footprint of the reactor vessel. Further, the present reactor is designed to permit a simple and effective means for addressing the structural problems induced by thermal effects by employing only inner and outer structural baskets and by providing a means to dense-pack multiple adsorbent layers between these baskets.
The teachings of the art are varied and inconsistent with respect to the design of radial flow reactors; particularly for vessels undergoing thermal cycling. Conventional cylindrical reactor designs typically include an internal assembly of at least two concentric porous wall baskets with the active material contained in the annular space formed between these baskets. The baskets and vessel shell generally share the same longitudinal axis. When multiple layers of active material are required in such radial flow reactors, the prior art employs additional structural porous separators between the layers of active material, i.e. the use of three or more concentric baskets. There are no teachings to achieve the dense-loading of adsorbents in radial flow type reactors operating under thermal cycling having baskets continuously affixed to the top of the reactor vessel. The patent art simply teaches pouring or dumping the active material through a sock or directly though top loading ports in the vessel.
U.S. Pat. No. 4,541,851 discloses in a first embodiment a vessel having two concentric layers of adsorbent, each layer contained between two concentric cylindrical grates. Three cylindrical grates are concentric about the same longitudinal axis as that of the vessel enclosing them. The intermediate grate is axially rigid and radially flexible while the inner and outer grates are axially flexible and radially rigid. All three grates are interconnected rigidly to the vessel shell at their upper end and interconnected rigidly to a solid floating bottom plate at their lower end.
In a second embodiment a vessel is described having three concentric layers of adsorbent and four permeable grates. The inner and outer grates are rigid in both the axial and radial direction and the two intermediate grates are rigid in the axial direction and flexible in the radial direction. All four grates are interconnected rigidly to the shell at their lower ends. Two or more layers of adsorbent can be used in this configuration. In both embodiments, the vessel has openings used for the filling and emptying of the adsorbent beds. Additional details are associated with this design are described by Grenier, M., J-Y Lehman, P. Petit, “Adsorption Purification for Air Separation Units,” in Cryogenic Processes and Equipment, ed. by P. J. Kerney, et al. ASME, New York (1984).
U.S. Pat. No. 5,827,485 discloses a vessel containing an annular adsorption bed which is bounded by inner and outer baskets. A single layer of adsorbent is taught which is contained between the two permeable concentric baskets, both of which are flexible in the axial direction and rigid in the radial direction. At least one of the baskets is rigidly fastened to the top end of the vessel. The inner basket is rigidly connected at its lower end to a bottom support member and further supported on a lower hemispherical cap of the shell by ribs arranged like a star. The outer basket is directly supported at its lower end by the bottom cap. Openings are present for apparently filling (and removal) of adsorbent although no discussion of the openings or the filling is found therein. Additional details are also described by U. von Gemmingen, “Designs of Adsorptive Dryers in Air Separation Plants”, Reports on Science & Technology, 54:8-12 (1994).
U.S. Pat. No. 6,086,659 discloses a radial flow adsorption vessel that has a plurality of grates, wherein at least one of the grates is flexible in both the axial and radial directions. The grates are rigidly attached to both the top of the vessel and to a bottom plate. The bottom plate may be floating or semi-rigidly or rigidly attached to the bottom head of the vessel. One or more intermediate grates are disclosed as a means to contain various layers of adsorbents within the vessel. The vessel has fill-ports for introducing and removing adsorbent, but no discussion of the filling process can be found.
German Patent No. DE-39-39-517-A1 discloses a radial flow vessel having a single layer of adsorbent contained between two concentric permeable grates, both of which appear to be rigid in both the axial and the radial direction. The outer basket is rigidly connected to the top end of the vessel and to a floating bottom plate. The inner basket is flexibly connected to the top end of the vessel through the use of an expansion bellows or a sliding guide. The lower end of the inner basket is connected rigidly to the floating bottom plate. The entire basket assembly is thus suspended from the top end of the vessel with the outer basket carrying the weight of the assembly and the adsorbent contained therein. Ports are used to introduce and remove adsorbent.
The patent art teaches many variations within basic design configurations wherein inner, outer, and/or intermediate baskets having various flexibilities are attached to the upper portion, bottom portion, or both portions of the vessel. The teachings for multilayer beds use an additional intermediate basket for each additional layer of material or adsorbent. These intermediate baskets are structural components that experience the loads and stresses induced by thermal cycling. Not only are the is the structural design and fabrication of the basket assembly made more complex by the presence of these intermediate baskets, but it is difficult to load adsorbents and to access and maintain components within each annular space. Such designs limit the loading of adsorbents to dumping, pouring or “sock loading” through ports on the top of the vessel resulting in a loose packing of materials subject to movement and settling during operation. The presence of intermediate baskets results in smaller volume spaces for loading active materials, further increasing the voids and decreasing packing density when active materials are poured or dumped into these spaces. As a result, the use of narrow or small-depth layers is limited when relying upon loose-pack loading methods. Thus, there is no clear teaching or direction in the art for the design of a radial flow reactor to mitigate or eliminate these problems.
The present radial flow reactor is designed such that the internal basket or basket assembly containing the bed of active material is rigidly supported at both the top and bottom ends of the vessel. The basket walls are axially flexible and radially rigid to minimize thermally induced movement and to control stresses and loads, thereby mitigating axial and radial buckling of the outer and inner baskets. A removable inner sleeve near the top of the inner basket can be temporarily removed to create a small open section in the basket. Such an opening enables the use of a rotating loading arm(s) to dense load either a single layer or simultaneously multiple layers of active material. The removable sleeve is then replaced for normal operation of the reactor. When it is desirable to separate adjacent layers of active material to prevent minor mixing of materials during loading such as when very thin layers are desirable, such separation is attained using a flexible, non-structural porous material placed at the interface between the layers.
The present invention not only enables uniform dense loading of active materials in either single or multiple layers, but also eliminates the need for additional structural baskets. The inventive radial bed reactor design permits dense loading, is more reliable to operate, and is less costly to manufacture.